Return a message when a 429 http response is received

[sandymcfadden]

Description

Currently, node-simperium only returns the response text from HTTP requests.
This adds a case for handling 429 responses from Simperium, which has an empty response body, and adds a response text which can then be used to handle this error specifically.

Now in clients if a 429 error is received you can check for it with something like
const errorMessage = error.underlyingError.message ?? '';
errorMessage === 'too many requests'; // 429 error

[dmsnell]

may sound silly given how simple this is right now, but I feel like a ladder of conditions would better set the stage for future changes here which might be likely. it's not the biggest issue, and there's a case to be made to let this be for now, but what are your thoughts about this construction instead?

xhr.onload = () => {
    if ( xhr.message === 429 && xhr.responseText = '' ) {
        return resolve('too many requests');
    }

    resolve(message);
}

you may find it silly for me to recommend changing a ternary into an if chain, but the early-abort here is key, that and the distinction that we're not really saying the message is one of two things, but rather that we're parsing the response and determining which of many possible outcomes we have.

[sandymcfadden]

the early-abort here is key, that and the distinction that we're not really saying the message is one of two things, but rather that we're parsing the response and determining which of many possible outcomes we have.

Your reasoning here makes total sense! It may not be 100% needed right now, but in the future, we may need to add more checks so I updated with your suggestion.

[codebykat]

LGTM